1. Field of the Invention
The invention relates to a method for regenerating electrically conducting adsorbents laden with organic substances which are heated by passing electrical current therethrough.
2. Description of the Related Art
Methods for removing organic gaseous substances of exhaust gas streams by adsorption on adsorbents, in particular, activated carbon, have been used industrially for many years, for example, in devices for recovering solvents. In most cases, the activated carbon is in the form of solid beds of granular material. Less frequently, fiber-shaped activated carbon in the form of mats, for example, nonwoven, woven or knit fabrics, is used, wherein the fabrics are wound to annular adsorbers or clamped into a frame construction. The entire process of exhaust gas cleaning by adsorption is carried out in general in three main steps: the loading step, the regeneration step, and the cooling step. During the loading step, the exhaust gas to be cleaned is passed through the adsorbent material and the organic substances are adsorbed during this process in the inner pore structures of the adsorbent material. During the course of loading, the loading front migrates in the flow direction through the adsorber. When the adsorbent material at the end of the adsorber, viewed in the flow direction, is loaded such that the clean gas concentration that exits the adsorber has reached a preset limit value, the loading process of this adsorber is terminated and the gas flow is switched to another freshly regenerated adsorber. For economic reasons, it is generally desirable to regenerate the laden adsorbent. However, many cases are known in which a regeneration is not possible or it is not economical. This is particularly the case when the concentration of the organic substances is very small, as is the case for intake air of buildings or the circulating air of air conditioning systems of buildings. In these cases, the laden adsorbent is disposed of and is replaced with new adsorbent material.
Since the adsorption is a reversible process, the adsorbed organic substances can be removed from the adsorbent material in a regeneration step by changing the equilibrium conditions. This can be achieved, for example, in that the adsorbent material is heated and the adsorbed organic substances are then removed from the adsorbent by means of a flushing gas stream. This method is, for example, employed in activated carbon adsorption devices which are regenerated with steam (water vapor) or inert gas. In both cases, the flushing gas serves at the same time as a heating medium for the adsorbent. Heating of the adsorbent and desorption of the organic substances are thus coupled in an un-separable way. Because of this indirect heating of the laden adsorbent with the flushing gas very large volume streams of the flushing gas are required. In the case of regeneration with steam, the steam, after exiting from the adsorber, must be condensed together with the desorbed organic substances that have been removed from the adsorbent. The resulting condensate must be regenerated in complicated processes, for example, by rectification such that water and organic substances are separated from one another. The inert gas regeneration in which the inert gas is generally nitrogen, is even more complex than the steam regeneration because the nitrogen is circulated through the adsorber and the nitrogen must then be cooled to very low temperatures in a condensation step in order for the desorbed organic substances to be condensed and for the residual loading of the adsorbent at the end of the regeneration step to be so low that the desired clean gas concentration can be maintained in the subsequent loading step.
Since the heating of the adsorbent by the inert gas as a result of its minimal heat capacity occurs only very slowly and since large flow velocity, which must be between 0.1 and 0.5 m/s, are required, enrichment or concentration factors (=maximum regenerating gas concentrations/crude gas concentration) of only approximately 40 can be achieved with this method . An already significant improvement can be achieved when the adsorbent is not heated indirectly by the flushing gas but directly by passing electrical current therethrough. In this case, as a result of decoupling of the processes heating and flushing, the flushing gas amount can be significantly reduced, and significantly higher enrichment factors can be achieved.
From German patent application 195 13 376 A1 a device for recovering organic solvents is known which is comprised of an annular solid bed filled with granular activated carbon and electrically heatable, which, for adsorption of the solvent, is first loaded with the exhaust gas stream and is subsequently loaded with the regenerating gas for desorption or regeneration. In this device, the outer and inner mantle of the annular solid bed is comprised of a metal grate which forms the electrodes. With this method, enrichment factors of 120 can be achieved and the required flushing gas volume stream can thus be reduced to 5-10% of the crude gas volume streams. With the very small specific gas amounts relative to the inflow surface area of the carbon bed against which the gas streams flow, the required uniform loading of the carbon with the flushing gas, which is required for a good regeneration, can be achieved only when it is carried out centrally in the interior of the adsorber, for example, by a distribution tube with numerous fine bores. A disadvantage in regard to the service life for loading and the quality of the clean gas concentrations in this method is that the regenerating gas flow itself is heated by the activated carbon and that, for a regeneration from the interior to the exterior, the carbon has a significantly reduced regeneration temperature in the innermost carbon layer as compared to the outer layers. As a result of this, the innermost carbon layer is regenerated only insufficiently, and therefore it is not possible to achieve very low clean gas concentrations during the loading step of, for example, a few xcexcg/m3 as required, for example, for the intake air for clean rooms.
From German patent document 41 04 513 C2 an adsorber is known which is also heated directly with electrical current and which is characterized in that the adsorbent material is in the form of fibrous activated carbon which is, for example, in the form of mats. One embodiment of such an adsorber is schematically illustrated in FIG. 1. The filter frame contains 5 to 15 layers of ACF (activated carbon fiber) fabric of 250 g/m2 per layer. At the top of the frame an electrical power supply of 220 V and 15 kW is provided and the bottom of the frame is connected to ground, while the vertical lateral frame parts are insulating. Crude gas (wide arrow) passes from the left to the right through the filter at 3,500 m3/h with a load of 100 mg/m3 at the inlet side and exits at the outlet side with a load of  less than 1 mg/m3 at 3,500 m3/h. The regenerating gas (narrow arrow) flows from the right to the left through the filter at 50 m3/h and exits to the left at 200xc2x0 C., 50 m3/h with a maximum load of 150,000 mg/m3.
Several of these frames can be switched together air flow-technically and electrically for the treatment of larger exhaust gas amounts in numerous sensible arrangements. Because of the very quick electrical heating of the fabric which can be achieved in less than a minute, very high enrichment factors of up to 1,500 can be achieved which are advantageous when, for example, solvents are to be recovered.
A disadvantage is that a large expenditure must be provided in order to distribute the regenerating gas uniformly across the filter surface. For example, the regenerating gas velocity relative to the filter surface in the above example of a 1 mxc3x973 m filter is only 0.01 m/s. For this flow velocity a pressure loss of only 10 Pa results across, for example, five layers of activated carbon fiber (ACF) fabric so that, at the time when the regenerating gas impacts on the filter, the gas must already be uniformly distributed. This can be achieved, for example, by a system of distribution pipes with numerous bores distributed across the height and the cross-section from which the regenerating gas exits as an open jet (FIG. 2).
For a large quantity of air to be treated, i.e., a plurality of filter frame modules, either a large number (3xc3x97number of filter frames) of distribution pipes must be installed or the spacing between a distribution pipe and the filter must be very large (0.87 m); this enlarges the construction volume of the apparatus to an uneconomical size.
Moreover, it is disadvantageous that in this method of electrical regeneration the desorption gas heating is also carried out by heat transfer from the electrically heated fabric onto the air, resulting in the above described negative results in regard to the efficiency of the exhaust gas cleaning method caused by the formation of a temperature gradient. When using activated carbon fiber fabric, this has, as shown in FIG. 3, an even stronger effect because of the minimal amount of employed carbon in comparison to granular carbon, where it is necessary for the treatment of the same amount of exhaust gas to employ an amount of active carbon that is more than 100 times greater. The result of such a great temperature gradient is that during regeneration the first fabric layers viewed in the regeneration direction, which are especially important for the fine cleaning of the exhaust gas, are not optimally regenerated which results in a shortening of the service life for loading and in a decrease of the achievable clean gas concentrations during the subsequent loading step. This negative effect is, of course, the stronger the higher the regenerating gas velocity.
As can be taken form the following table, a further disadvantage at higher desorption gas velocities is that the required electrical heating power increases drastically with increasing desorption gas velocity. This has the consequence that, for economical reasons, the regenerating gas velocities are kept as low as possible for electrically regenerated adsorbers made of activated carbon fiber material, which in turn, requires the installation of a complex regenerating gas distribution system.
Finally, in this method it is a disadvantage that the control of the temperature of the activated carbon fabric, which is also of utmost importance for safety-technological reasons for the practical use of the method, is very difficult. The reason for this problem resides inter alia in that the thermal conductivity and the heat capacity of the activated carbon fiber material is very low so that conventional methods of temperature measurement, for example, by means of electrically insulating thermoelements, react only with great time delay and, moreover, the measured value is strongly flawed because it is dependent on how good the (accidental) mechanical contact between the thermoelement and the fabric is. Since the heating duration of the fabric from room temperature to, for example, 200 xc2x0 C., requires only approximately one minute and, moreover, the electrical resistance of activated carbon fabric has a negative temperature coefficient, there is a great risk of overheating of the activated carbon fabric connected with this method.
In both known electric regeneration methods of adsorbents a cooling step follows in which the adsorbent is cooled to a temperature which is required for the subsequent loading step by passing air or inert gas through the adsorbent.
It is an object of the present invention to provide a method for electrical regenerating electrically conducting adsorbents, in particular, of activated carbon fiber fabrics, which, on the one hand, provides the benefits of electrical regeneration in comparison to conventional regeneration methods, i.e., inert gas regeneration and steam regeneration, such benefits including:
very short desorption periods;
very short heating periods;
very great enrichment factors for the regeneration;
if needed, regeneration temperatures up to 300xc2x0 C.;
in the case of fiber-shaped adsorbents, extremely quick kinetics of the material transfer from the gas to the adsorbent phase;
good economic efficiency because in the electrical regeneration methods only the activated carbon and the regenerating gas but no pipelines, containers, or other apparatus parts must be heated;
but does not exhibit the disadvantages of conventional electrical regeneration methods for adsorbents as described above and, accordingly, in particular:
makes possible also large desorption gas velocities of substantially more than 2 cm/s without the connected load for heating the adsorbent being substantially greater than for desorption gas velocities under 1 cm/s; this is particularly desirable when the regenerating gas is air and this regenerating air must not be further cleaned, for example, in the case of cleaning the intake air of buildings from the gaseous substances contained in the ambient air;
requires no complex device for the uniform distribution of the regenerating gas across the inflow surface of the filter so that, on the one hand, the apparatus expenditure for this distribution device is eliminated and, on the other hand, the space requirement of such filters in comparison to the conventional configuration is considerably reduced;
provides an intrinsically safe regeneration method with regard to overheating or burning of the activated carbon fiber, i.e., no temperature control of the electrical power is required.
In accordance with the present invention, this is achieved in that alternatingly and temporally successively with respect to time the adsorbent is heated by passing electrical current therethrough, wherein no flushing gas stream is guided through the adsorbent, and, subsequently, with the electrical current being turned off, a flushing gas stream is guided through the adsorbent so that at the same time the adsorbed organic substances are removed and the adsorbent is cooled.
Preferably, the two steps of heating and desorbing/cooling are performed temporally successively several times.
Advantageously, the step of heating is carried out for a preset period of time.
The preset period of heating is calculated such that it corresponds to the time which is minimally required in order to heat, for a preset electrical heating power, the completely unladen adsorbent to a predetermined maximum temperature.
Preferably, the empty pipe velocity through the adsorbent is 0.1 to 0.5 m/s.
The adsorbent is preferably comprised of fiber-shaped activated carbon in the form of nonwoven, woven or knit fabrics.
The preset periods of time are selected such that the adsorbed mixtures are successively desorbed and collected according to their volatility.